Control systems for driving a brushless DC motor without using a position sensor such as a hole IC are classified into two (first and second) systems. In the first system, an energization (current supply) angle is less than 180°. In the second system that drives the brushless DC motor with a sinusoidal wave voltage, the energization angle is 180°.
The first control system, in which the energization angle is less than 180°, detects a zero cross point of an induced voltage which is developed in a winding wire in an energization pause interval, and performs position estimation to control the driving of the brushless DC motor. The first control system can be realized with a simple logic.
The second control system that drives the brushless DC motor with the sinusoidal wave voltage in which the energization angle is 180° cannot conduct the position estimation based on the induced voltage that is developed in the energization pause interval. For this reason, the second control system performs calculation based on the winding voltage, the winding current, and the motor constant at a high speed to estimate the position, thus controlling the driving. The second control system causes very small torque ripple. Thus, noises and vibrations can be suppressed, because the sinusoidal wave voltage is applied to the system. However, a high-precision current sensor is required, and a high-performance microcomputer is also required to conduct the high-speed calculation.
In the above sensorless control, when the motor is under suspension, or in the region where the rotation speed is low, because the position cannot be detected, forced commutation is executed so that start is performed as a synchronous motor to switch over to the sensorless control. For example, as shown in FIG. 22, in the case of conducting the sensorless control by a drive signal whose voltage waveform is rectangular waveform when the energization angle is less than 180°, start is performed by the forced commutation of the drive signal having the rectangular waveform even in the forced commutation at the time of start (for example, refer to IP 58-29380A).
For example, in a motor that is mounted in a vehicle, a difference between the highest and lowest operating environment temperatures is very large. When the atmospheric temperature rises, the winding resistance of the motor increases, and resistances of a semiconductor device and a wiring in an electronic circuit portion that mainly includes a drive device also increase. As a result, even if a voltage that is applied to the motor has the same level, a current that flows in the motor is reduced, thereby also reducing the developed torque.
Accordingly, in the case where the developed torque is reduced by a rise of the atmospheric temperature when the forced commutation is performed at the time of start, it is likely that a rotor does not follow the commutation speed of the drive circuit side, and fails to start. In order to avoid the above problem, it is necessary to apply the voltage of a sufficient level to the motor so as to prevent the deficient torque at the time of increasing the temperature.
However, it is assumed that an applied voltage is determined in response to the temperature rising time as described above. Then, when the atmospheric temperature is about a room temperature, or is lower than the room temperature, the winding resistance of the motor and the resistances of other circuit portions are reduced. As a result, the current reversely increases, and the developed torque increases. In the forced commutation that is caused by the drive signal whose voltage waveform is the rectangular waveform in which the energization angle is less than 180°, the vibrations of the motor which is attributable to the torque ripple increase, to thereby generate an abnormal noise at the time of start.
Also, the motor that is mounted in, for example, a vehicle, particularly in an engine compartment, is very severe in the operating environment, and the magnetic flux of a rotor magnet is deteriorated under the high-temperature environment. As a result, it is supposed that the amplitude level of an induced voltage that is developed in the stator winding wire at the time of rotation is also decreased, and the control cannot shift to the sensorless control. The magnetic flux is deteriorated when a rotor magnet is rusted while the motor is used for a long period of time.
In JP 2005-137069A, when it is determined from synchronized operation that the shift to the sensorless control fails, the applied voltage at the time of forced commutation increases, or a rotation speed for switching over to the sensorless control increases to retry the shift.
However, in the above system, when the failure of shifting to the sensorless control is caused by the environment or the secular change, the problem cannot be essentially solved by merely changing the control mode, and there is a risk that the restart is repeated. Then, when the applied voltage increases or the rotation speed increases to execute the forced commutation by a conventional rectangular waveform drive signal, the noise at the time of driving becomes large. As a result, a vehicle passenger may feel uncomfortable.